Answer the following questions : (a) A steady current flows in a metallic conductor of non-uniform cross-section. Explain which of these quantities is constant along the conductor : current, current density, electric field and drift speed ?
(b) Is Ohm's law universally applicable for all conducting elements ? If not, give examples of elements which do not obey Ohm's law.
(c) A low voltage supply from which one needs high currents must have low internal resistance, why ?
(d) A high tension `(HT)` supply of say `6 kV` must have a very large internal resistance. why ?

Explanation

(a) Only current will be constant along the conductor. The rest of the quantities depend on the area of cross section of the conductor inversely.
(b) No, Ohm.s law is not universally applicable for all the conducting elements. There are elements that conduct electric current without obeying Ohm.s law. Examples of non-ohmic elements include vacuum diode tube and semiconductor diode.
(c ) The maximum current drawn from a source `= (E )/(r)`. Since the maximum current is inversely proportional to the internal resistance of the supply, therefore for high value of current, the supply must have a very low internal resistance.
(d) If the circuit of HT supply is short-circuited (accidentally), the current drawn will exceed safety limits if internal resistance is not large. Thus, to limit the current to a safe level, a HT supply has a very large internal resistance.